Why does the simple deform modifier not bend when created or strechted on the Y axis?
There are two axes to take into account when using the Simple Deform modifier.

The axis of deformation, set in the modifier, around which, in 'Twist' and 'Bend', vertices rotate.
The axis of extent, which determines, according to the fraction of the object's bounding box along it, either side of the origin, how much the vertices are rotated.

The examples below have their 'Bend' deformation axes set to X, Y and Z:

as you can see, the axes of extent are Z,Z, and X.
I wish you could set both axes in the modifier. Personally, I always wind up using an Empty as an axis/origin object, and fiddle with it until it works.